(a) Field of the Invention
This invention relates to catalyst-supported particulate filters.
(b) Description of Related Art
Exhaust gases from diesel engines using fuels containing diesel oil as a main component and exhaust gases from gasoline engines using fuels containing gasoline as a main component to burn them under fuel-lean conditions are known to contain not only hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), but also particulates (suspended particulate matters containing carbon particles). To restrain particulates in exhaust gas from being exhausted to the atmosphere, a filter for trapping the particulates is disposed in an exhaust gas passage of such an engine. However, when the amount of particulates deposited on the filter becomes large, this causes deterioration of engine power and fuel efficiency. Therefore, in this case, it is necessary to burn the deposited particulates as appropriate to remove them from the filter.
To effectively burn such deposited particulates (ignite the particulates at relatively low temperature and burn them out in a short period of time), a catalyst layer containing alumina carrying platinum (Pt) as a catalytic metal is formed on the walls of exhaust gas channels in the body of such a filter. Such Pt-carried alumina is effective in burning particulates as well as converting HC and CO, but there have been recently developed catalyst materials for filters capable of more efficiently burning particulates.
Published Japanese Patent Application No. 2006-326573 describes the use of such a catalyst material in a filter. The catalyst material is a material in which a catalytic metal, such as Pt, is carried on a mixed oxide (composite oxide) containing cerium (Ce), zirconium (Zr) and a rare earth metal R selected from among praseodymium (Pr), neodymium (Nd) and lanthanum (La). The published document also describes that the content of R in the mixed oxide is preferably 2% to 11% by mole, both inclusive. Since such a mixed oxide contains Ce, it has oxygen storage/release capacity so that oxygen released from the mixed oxide promotes ignition and burning of particulates.
Published Japanese Patent Application No. 2007-54713 describes the use of another catalyst material in a filter. The catalyst material is a material in which a catalytic metal is carried on mixed oxide ZrRO particles containing Zr and a rare earth metal R selected from among ytterbium (Yb), Nd and scandium (Sc) and having an R content of up to 18% by mole and mixed oxide CeMO particles containing Ce and a rare earth metal M selected from among samarium (Sm) and gadolinium (Gd). The ZrRO particles have oxygen ion conductivity to thereby release active oxygen but their oxygen release mechanism is different from that of CeZr-based mixed oxides as disclosed in Published Japanese Patent Application No. 2006-326573.
Specifically, CeZr-based mixed oxides have high oxygen storage capacity and release active oxygen by the valence change of Ce ions. On the other hand, the ZrRO particles have oxygen ion conductivity, i.e., so-called oxygen pumping function, and, when having both of high-oxygen concentration sites and low-oxygen concentration sites on the particle surfaces, transport oxygen ions from high-oxygen concentration sites to low-oxygen concentration sites and then release them as active oxygen.
Therefore, for the ZrRO particles, when a small ignition site causing particulates to burn is formed on their surface and then falls into lack of oxygen, oxygen is transported from other high-oxygen concentration sites. Thus, the burning is continued and the burning region on each particle can be easily extended from the ignition site to the surrounding sites.
Published Japanese Patent Application No. 2007-83224 describes the use of still another catalyst material in a filter. The catalyst material is a material in which a catalytic precious metal is carried on ZrRO having oxygen ion conductivity as stated above and alumina.
Published Japanese Patent Application No. 2007-98200 describes a method for manufacturing a catalyst material for converting exhaust gas components. In this method, an excessive amount of aqueous ammonia is first added to a mixture of an aqueous solution of aluminium nitrate and an aqueous solution of lanthanum nitrate to precipitate a first hydroxide containing Al and La. Next, a mixture of an aqueous solution of cerium nitrate and an aqueous solution of zirconium nitrate is further added to the above mixture to precipitate a second hydroxide containing Ce and Zr over the precipitate of the first hydroxide. Then, the obtained precipitate of the first and second hydroxides is filtrated, dried and calcined. According to this method, a catalyst material is obtained in which each of activated alumina particles containing La forms a core and the whole surface of the core is covered with a Ce—Zr mixed oxide serving as a shell.
Published Japanese Patent Application No. 2005-313029 discloses that metal oxide particles each including a core containing a large proportion of zirconia and a shell containing a large proportion of ceria are used for an exhaust gas purification catalyst, wherein the core and shell are each composed of a plurality of primary particles, the diameter of the primary particles of the shell is smaller than that of the primary particles of the core and each of the primary particles of the core or the shell is made of a ceria-zirconia solid solution or contains yttrium (Y).